Generation of DC voltages has to explained

1. Measurement Of High Dc Voltage
Contents
•Series resistance micrometer
•Resistance potential divider
•Generating voltmeter
•Sphere gaps
•Conclusion and Reference
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3. Series resistance micrometer
• Resistance (R) :
– Constructed with large wire wound
– Value: Few hundreds of Mega ohms –Selected to give (1-10μA) for FSD.
– Voltage drop in each element is chosen to avoid surface flashovers and discharges
(5kV/cm in air, 20kV/cm in oil is allowed)
– Provided with corona free terminals.
– Material: Carbon alloy with temperature coefficient of 10-4/oC .
– Resistance chain located in air tight oil filled PVC tube for 100kV operation with
good temp stability.
• Mircoammeter – MC type
• Voltage of source, V=IR
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4. • Impedance of the meter is few ohms. i.e., very less compared to R so the
drop across the meter is negligible.
• Protection: Paper gap, Neon Glow tube, a zener diode with series
resistance – Gives protection when R fails.
• Maximum voltage: 500kV with  0.2% accuracy.
• Limitations:
– Power dissipation & source loading
– Temp effects
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5. • A very high resistance in series with a micrometer.
• Current through R is measured using micrometer.
• Voltage of source, V = IR
• The resistance is constructed from a large no. of wire wound resistors in
series.
• Can be operated up to 500kV (D.C)
• Accuracy = ±0.2%
• Selection of R value:
– Current allowed: 1 to 10A
– Corona free termination
– Temp. coefficient<10-4/0C : Carbon Alloy
– Placed in airtight, oil filled PVC tube to maintain temp. stability
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6. Resistance potential divider
• It uses electrostatic voltmeter.
• Can be placed near the test object which might not always be confined to
one location
• Let, V2-Voltage across R2
• Sudden voltage changes during transients due to:
– Switching operation
– Flashover of test objects
– Damage due to stray capacitance across the elements & ground
capacitance
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8. Generating voltmeter
• Generating principle is used where direct loading or direct
connection is to be avoided.
• Generating voltmeter: A variable capacitor electrostatic voltage
generator.
• It generates current proportional to voltage under measurement
• This arrangement provides loss free measurement of DC and AC
voltages
• It is driven by synch. motor, so doesn’t observe power from the
voltage measuring source
• The high voltage electrode and the grounded electrode in fact
constitute a capacitance system.
• The capacitance is a function of time as the area A varies with time
and, therefore, the charge q(t) is given as,
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9. Schematic of generating voltmeter
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10. • Fig. shows a schematic diagram of a generating voltmeter
which employs rotating vanes for variation of capacitance
• High voltage electrode is connected to a disc electrode D3
which is kept at a fixed distance on the axis of the other low
voltage electrodes D2, D1, and D0.
• The rotor D0 is driven at a suitable constant speed by a
synchronous motor.
• Rotor vanes of D0 cause periodic change in capacitance
between the insulated disc D2 and the high voltage electrode
D3.
• Number and shape of vanes are so designed that a suitable
variation of capacitance (sinusoidal or linear) is achieved.
• The a.c. current is rectified and is measured using moving
coil meters. If the current is small an amplifier may be used
before the current is measured.
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11. • Generating voltmeters are linear scale instruments and
applicable over a wide range of voltages.
• The sensitivity can be increased by increasing the area
of the pick up electrode and by using amplifier circuits
Advantages:
– scale is linear and can be extrapolated
– source loading is practically zero
– no direct connection to the high voltage electrode.
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12. Sphere Gaps
• Applications:
– Voltage Measurement (Peak) - Peak values of voltages may be measured from 2 kV up to
about 2500 kV by means of spheres.
• Arrangements:
1. Vertically with lower sphere grounded (For Higher Voltages)
2. Horizontally with both spheres connected to the source voltage or one sphere grounded
(For Lower Voltages).
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13. • The arrangement is selected based on the relation
between the peak voltage, determined by spark over
between the spheres, and the reading of a voltmeter
on the primary or input side of the high-voltage source.
This relation should be within 3% (IEC, 1973).
• Standard values of sphere diameter are 6.25, 12.5, 25,
50, 75, 100, 150, and 200 cm.
• The effect of humidity is to increase the breakdown
voltage of sphere gaps by up to 3%.
• Temperature and pressure, however, have significant
influence in breakdown voltage.
• Breakdown Voltage under normal atmospheric
conditions is, Vs=kVn where k is a factor related to the
relative air density (RAD) δ.
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14. • The relation between the RAD(δ) and the
correction factor k:
• Under impulse voltages, the voltage at which there is a 50%
breakdown probability is recognized as the breakdown level.
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15. • Factors Influencing the Spark over Voltage of Sphere Gaps
i. Nearby earthed objects,
ii. Atmospheric conditions and humidity,
iii. Irradiation, and
iv. Polarity and rise time of voltage waveforms.
• The limits of accuracy are dependant on the ratio of the spacing d to the
sphere diameter D, as follows:
– d < 0.5 D Accuracy = ± 3 %
– 0.75 D > d > 0.5 D Accuracy = ± 5 %
• For accurate measurement purposes, gap distances in excess of 0.75D are
not used
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16. • Conclusion
From these we conclude that how to measure
varies measurements of how to generate high
DC voltage in power system engineering.
• References
“High voltage engineering ” by M S Naidu and
V Kamaraju, Tata McGraw Hill Education, 5th
edition.
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